Drilling mud treatment



GR. 2 953,1s696 a... was-.-

Patented Oct. 12, 1943 UNITED STATES PATENT OFFICE to Union Oil Companyof California, Los Angeles, Calif., a corporation of California NoDrawing.

Application February 16, 1942,

Serial No. 431,058

7 Claims. (Cl. 252-85) This invention relates to the preparation,conditioning and recovery of drilling muds in genera] and relatesparticularly to the re-conditioning or reclaiming of well drilling mudswhich have become contaminated with cement during cementing operationsas in oil well drilling. This application in a continuation in part ofmy copending application Serial No. 256,304 filed Feb. 14th, 1939, whichin turn is a continuation-inpart of my application Serial No. 150,745,filed June 28, 1937 which in turn is a continuation-inpart of myapplication Serial No. 130,735, filed March 13, 1937. This applicationis also a continuation-in-part of my application Serial No. 213,214,filed June 11, 1938.

Mud which has been contaminated with cement is termed "cement cut andsuch cement cut mud is injured materially with respect to many of thefunctions which it is required to perform.

A poor quality mud forms a thick permeable mud cake upon the walls orsurfaces of penetrated formation and allows at the same time arelatively large loss of water from the drilling mud by seepage orfiltration into the penetrated formation.

In oil well drilling operations, for example, this may result incontamination of the adjacent penetrated producing oil sands withsubsequent impaired oil production rates. Also, certain formations aregreatly weakened structurally by the ingress of water.

The structural strengths of most formations that are penetrated by thedrill are sufficient to prevent the walls of the hole from sloughing orcaving. However, many such formations lose structural strength when wetwith water and when so wet cave or slough into the hole. Frequently suchaction causes seizing of the drill pipe or tools so that they cannot bemoved and costly fishing jobs result. It is, therefore, of greatimportance to prevent the loss of water from the drilling mud to theformations drilled. The mud cake formed by such poor quality muds mayalso at times be of sufllcient thickness to obstruct the movement andeven cause seizure of the drilling tools in the bore hole.

A high quality mud, on the other hand, forms a thin but highlyimpervious mud layer upon the penetrated formation surfaces which allowsa relatively low loss of water from the drilling fluid to the formation.The high quality mud appears to have good colloidal properties which notonly contribute to the before mentioned desirable qualities but alsoserve to impart the requisite thixotropic and plastic characteristicsfor the proper support of the drill cuttings so that they can be liftedreadily without settling from the bore hole by the circulated medium.

The desirable quality of drilling mud has therefore, in this respect,been defined as that property of the mud which allows a minimum loss ofwater to the formation, while at the same time forming an impervious mudlayer or cake of minimum thickness upon the penetrated formationsurfaces.

A further desirable quality in drilling muds is a sumciently lowviscosity to permit easy pumping and circulation.

The relative performance qualities of muds as they relate to the abovementioned formation penetration and mud cake formation characteristlcscan be determined by subjecting natural and artificial formation samplesto contact with the drilling muds under question under actual drillingoperations and under correlated test conditions simulating actualdrilling well conditions but where the quantities of water penetratingthe artificial formations and the thickness of the mud cakes can bereadily measured. I find furthermore that these characteristics can bedetermined by a simple filtration test as here inafter described. Thesetest indications were obtained and correlated with the actual resultsobtained in drilling wells as explained hereinafter. The viscosities ofthe muds are determined as hereinafter described.

It has been discovered that drilling muds which have been contaminatedby contact or admixture with cement during cementing operations in awell usually have subsequent to such contact, poor performancecharacteristics in accordance with the quality definition givenhereinbefore. Moreover, such cement cut muds often have extremely highviscosities which causes difliculty in circulation, imparts gas-cuttingtendencies and prevents the proper release of cuttings.

Cement, as is well known, contains a complex mixture of oxides ofcalcium, magnesium, iron, aluminum and silicon. Of these compounds,calcium oxide or lime is present in greatest quantity. Calcium sulphatesor gypsum may also be present in small quantities to act as acontrolling agent in the setting reaction rate. Therefore, when ceme ismixed with an excess of water as in the case of contact with drillingmud in cementing operatioiis, the soluble portions of the cementdissolve in the water, forming therein a fairly high concentration ofdi-valent positive calcium ions associated with negative hydroxide ions.

Minor quantities of the di-valent magnesium and tri-valent ions ofaluminum and iron from the cement usually are also present in solutionin the water in the drilling mud.

The presence of these ions appear to cause a flocculation of thecolloidal material in the mud and thus to deprive it of its sentialformation sealing properties. At the same time the visccsity of the mudis greatly increased by these impurities to the extent that circulationof the mud becomes diflicult and gas-cutting is facilitated.

satisfactory muds tend to deposit a thick, tough mud cake. I havediscovered, therefore, that in treating a drilling mud to give itpropertie insuring satisfactory performance that it is desir- It hasbeen discovered that in general cement 5 able that the treated mud, whentested as above cut muds or other similarly contaminated muds described,yields a total filtrate of not over 40 to can be reclaimed by treatingthem with carbon 45 ml. at an average rate during the secondhalfdioxide. It has f ermore been discover hour period of not over 25 to30 ml. per hour and dire-quantity of the reagent used in such treatthatthe deposited cake be preferably less than ment should be such as tobring the physical 1o one-quarter inch in thickness and preferably ofcharacteristics of the mud as above described asoft gelatinous texture.within certain ranges of value which I have It is desirable that theviscosity of the mud fou d to be essential in pragtice, should be lessthan 45 seconds as determined by The most important physical property ofthe the Marsh funnel viscometer, a description of mud is its ability toseal formations against inwhich can be found in an article by H. N- M rfiltration of water, and as a result of extensive n i roperties andTreatment of Rotary research I have found that this property can be pping in the Transaction of the measured by a simple filtration test. Theseal- A. P. I. M. E. Petroleum Development and Teching properties of themud I find to depend on the 5. P e 237 ct qn 1. I find that thecharacter of the mud cake or filt r cake formed funnel ViSCOSities inexcess 0f 45 lead to impaired when the mud is pressed against a membraneor circulation and increased danger of gas cutting. filter permeable towater, rather than on the It has been discovered that in general cementcharacter of the membrane or filter, Consecut muds 01 other similarlycontaminated muds t I find th t i testing th sealing propwhich are oflow quality with respect to their mud erties of mud I may use eithersamples of the cake forming characteristics and ability to preaotualformatio n e n d a filter paper a vent loss of water to formation can bereclaimed the permeable membrane, and in either case obby treatment W thr n dioxide. tain test values showing the same general rela- In thepractice of this invention, n t ro ary tionships between the mudstested. In practice, method of oil well drilling for x the r t- I testthe sealing properties of muds by placin reagent is added to thecirculatin mud 600 ml. of mud in a 3-inch internal diameter stream atany c n ni nt point such as at a point cylindrical filter provided atits lower end with l' c t 0 t e d pump suction inlet in the a ReeveAngel No. 214 filter paper and filter the mud sump. Thorou h mixture ofthe thus intromud under air pressure of 100 pound per square ducedreagent may be assured by rapid recirculai h tion of the mud from themud sump through the The test measurements consist in determining SpareS h p mp. In the case of empl ying carthe total filtrate water obtainedduring the first hon ox de as the treating gent, it is added to hour ofpressure of titration, the average rate of the mud y lowing it into themud stream at or filtration during the last half of this hour and thenear the said mud p mp suction inlet. During thickness and texture ofthe mud filter cake 40 treatment. m samples are taken from the cirformedduring the hour of filtration. Numerous 91119191118 mud stream atfrequent intervals and correlations between such tests and actualdrilltested in the before-mentioned fi fl fl testing ing experience withthe same muds have shown apparatus to de ermine when the treatment hasthat the muds yielding a total filtrate f l t a effected the requireddegree of recovery of the de- 30 1, using 00 m1, sample in an n n n.sirable formation penetration and mud cake tration period at atemperature of 80 F. are very forming c aracteristics. satisfactory. Onthe other hand, muds yieldinga An example of the prac ca application andtotal filtrate in excess of 45 ml. under similar results of the processof this invention is illuscircumstances have been found dangerous touse, trated by 8 number of p l te t d ta in Table I. and in particularwhen muds of this character 5 The method employed for making theseperare used in drilling through formations which formance tests was asoutlined above. In this are readily softened by penetration of water,instance, however, sand was used in place of cave-ins frequently occur.Again, mud displaythe filter paper and the duration of each test ingaverage filter rates in excess of 25 or 30 ml. was thirty minutesinstead of one hour. The per hour for the last half hour of filtrationhave drilling mild to which the Process and te s Were been foundobjectionable in use. Also, a mud applied was cement-cut mud from theDominguez which is satisfactory for drilling operations will field,California; the test formation comprised under conditions of this testdeposit a filter cake Temblor sand from Kettleman Hills. The temof notover one quarter inch in thickness and of a perature and pressure of thetest mud was F. soft plastic or gelatinous texture; whereas ungo andpounds per square inch, respectively.

Table I Water penetration Tests on mud through test forma- CO! tion ml.Thick added Test No. to

per gram 0! adjust Rate durmud cake solidsinmud pH weight Fvuiiggel pH1331211111 1511115111.

perhour LIL/craft. Seconds Inches 0 0 00.0 12.3 78.0 104.0 0.30 0 00.020 10.0 49.0 52.0 0.25 a 0 68.0 25 12.3 32.0 38.4 0.25 12 65.1 20 10.029.0 33.0 0.125 20 cs2 20 10.3 20.0 29.0 0.125

"2. coiiPoemoi-ig Search Hoom The improvement in characteristics of thecement-cut mud is evidenced by the decreased viscosity as shown incolumn 5,principally by the data of the last two right-hand columns ofTable I, where it is apparent that the water penetration rate and mudcake formation thickness characteristics of the mud were improved bytreatment with either sodium carbonate or carbon dioxide alone butespecially by the combined treatment by both sodium carbonate and carbondioxide. Large numbers of tests such as those in Table I correlated withactual well drilling experiences and results of the application of theinvention thereto have shown that the combination of sodium carbonatetogether with sufficient quantity of carbon dioxide, to adjust thehydro- "gen ion concentration (acidity or pH value) to a pH valuebetween 8 and 11.5 averaging about 10, or the use of sodium bicarbonatealone for this purpose produce the most satisfactory results.

As a valuable alternative to the process as described above, I may addthe aforementioned reagents to fresh mud not only to improve itsimmediate quality but also to immunize the mud against futurecontamination with cement. Thus, for example, I may add to a mudsufficient carbon dioxide to precipitate the calcium that wouldotherwise be introduced during future cementing operations andsubsequent operations of drilling through the resultant cement plug. Themud may be thu immunized by prior addition of suitable reagents as abovedescribed or combinations of such reagents.

An example of the practical application of the process of this inventionis illustrated by results obtained by its application to drilling wellsand by correlated typical test data as outlined in Table I.

For example, trouble may be encountered in removing and insertingdrilling tools in a drilling well subsequent to cementing operationswhich have contaminated the circulating mud. Sticking of drilling toolsin a well is believed to be caused by the penetration of formationssubject to swelling or caving when they become permeated with water fromthe drilling mud. Sticking of the tools is also attributed to theobstructing of the drilled hole with the excessively thick mud cakeformed by drilling muds of poor quality. Whatever the reasons for thesediificulties in drilling may be, they are believed to be associated withthe use of a drilling mud of poor quality as defined hereinbefore whichallows a substantial quantity of the water to penetrate the formationsand which causes the formation of a thick mud cake. When drillingthrough a low pressure producing formation a drilling mud of poorquality allows the contamination of the oil sands with water from thedrillingmud which acts to reduce the permeability thereof and results inreduced future rates of production.

When, as stated hereinbefore, trouble is encountered in removing andinserting drilling tools in a drilling hole, a sample of the circulatingdrilling mud from this well may be tested by subjecting an artificial ornatural test formation to contact with the said mud sample underpressure in accordance with the method described hereinbefore. If themud has been seriously contaminated with the cement the test will showthat it has lost a considerable portion of its desirable formationsealing properties. The mud sample thus tested and found to becontaminated with cement is then treated with carbon dioxide gas. Thethus treated mud sample is then again subjected to the test and isordinarily found after such treatment to have been restored to itsproper formation sealing value. The circulating drilling mud in the wellfrom which the test mud sample is taken is then treated in accordancewith the data obtained from the before described test and the followingdrilling operations will prove to be substantially freer from drillingtool sticking tendencies.

An example of the valuable savings which may be effected by theapplication of this invention is evident by the following: 1200 barrelsof cement out mud which would ordinarily be discarded was withdrawn froma drilling well in Dominguez field, California, and placed in a storagetank. This cement cut mud, upon withdrawal, was a jelly-like massentirely unfit for drilling operations, and upon testing prior totreatment showed a viscosity too high to permit funnel viscositydeterminations to be made successfully. Eighty- I five pounds of carbondioxide gas was mixed into this mud by blowing it into the suction inletof a mud circulating pump connected to the said storage tank resultingin a reduction of the funnel viscosity to 25. The resultantreconditioned mud was then suitable for further utilization in drillingoperations.

The muds to which this invention is applicable comprises in generalwater suspensions of clayey solids usually associated with colloidal orcolloid forming substances in sufiicient quantity to impart permanenceto the suspension, the proper viscosity and plasticity characteristicsfor carrying cuttings and the requisite formation sealing properties.Such muds may for example be one of the well known Wilmington Slough,Dominguez Slough, Coalinga Red, Frazier Mountain or other muds or blendsthereof of recognized merit. The muds may be of the bentonite type ormay contain additions of bentonite or other colloidal quantity ofcement, or its equivalent, will then respond more readily to subsequenttreatment and the thus treated mud is usually found to havecharacteristics which are superior to those of the original mud and alsosuperior to those obtainable by the aforementioned treatment alone.

Furthermore, I have discovered that the improvement to be gained byfirst cement cutting the mud followed by the aforementioned treatment isnot confined to those muds which are initially resistant to thetreatment but that it extends to many other muds ordinarily responsiveto such treatment and also to those muds ordinarily considered to be ofgood quality.

The reason for this improvement is not entirely understood but itappears to be the result of an initial partial flocculation caused bythe cement cutting treatment followed by a partial defiocculation causedby the "subsequent treatment.

Best results appear to be obtained by first treating the mud with aquantity of cement, or its equivalent chemical such as sodium or calciumlam-n...-

hydroxide, in sufilcient quantity to partially flocculate the mudwhereupon it assumes the consistency of a semi-gel followed by treatmentwith carbon dioxide to partially deflocculate the mud and thus greatlyreduce the viscosity. The thus treated mud is found to exhibit asubstantially reduced viscosity and filter rate.

It thus appears that in order to reduce the viscosity and filter rate ofa mud by a chemical treatment it is first desirable to flocculate themud partially into a gel or a semi-gel form, in which state theparticles are larger than they were originally. Then the addition of areagent which has some peptizing action to deflocculate the enlargedparticles results in a decrease of .viscosity below the original.

It is preferable to first add the flocculating agent such as cement,then the deflocculating agent such as carbon dioxide in this order ashereinbefore described because the physical and chemical charges arethereby almost immediately efiected. However, the order of treatment maybe reversed or the treating agents added simultaneously but when thetreatment of the mud is performed in this manner the mud generallyreacts very slowly to the treatment requiring in the order oftwenty-four hours of agitation to complete the desired chemical andphysical changes, until the chemical treatment reaches substantialcompleteness the mud has a gel-like consistency which makes its pumpingand circulation through the drill stem impractical. Hence, it isobviously impractical in the field to chemically treat the mud except inthe before described order, that is, first with the flocculating agentfollowed by the deflocculating agent.

The flocculating agents which have been found useful are sodiumcarbonate, sodium hydroxide, sodium chloride, lime, calcium chloride,Portland cement, sodium silicate, magnesium chloride, aluminum chloride,aluminum sulphate, alum, iron chloride, iron sulfate, sodiumorthophosphate, sodium pyrophosphate, sodium oxalate, sodium tartrate,sodium aluminate, sodium ammonium phosphate, sodium citrate, sodiumborate and sodium fluoride.

In general the quantity of reagent necessary to use may be determined bythe stoichiometrical relationships of the reactions involved, based onan analysis of the water associated with the mud or a knowledge of thetype of contamination. In other instances such as have been describedherein the quantity or reagent is adjusted to give a certain pH value;thus CO2 is added to a cement cut mud until the pH falls between 81l.5.In any case the reagent may be added in a quantity such that the desiredcharacteristics of the mud, such as yield point, viscosity and filterrate are obtained by tests as described hereinbefore, for example untilthe filter rate is reduced to 45 mL/hr. and/or until the funnelviscosity is approximately 28.

The foregoing is illustrative and not to be taken as limiting theinvention, which may include any method which accomplishes the samewithin the scope of the appended claims:

I claim:

1. In the drilling of a well with the circulation of a drilling fluidwhich becomes contaminated with cement, the method of conditioning thecement-cut drilling fluid which comprises treating the drilling fluidwith W drilling mud having a lower filtration characteristic than saiduntreated mud.

3. A method for improving the filtration characteristics of an untreateddrilling mud which had not been previously employed as a drilling mud inconnection with the drilling oi! wells which comprises treating the mudwith a sufficient quantity of calcium hydroxide to effect flocculationof the mud to a semi-gel consistency and thereby initially producing amud having a reduced filtration characteristic and subse-' quentlytreating the flocculated mud with a sufficient quantity of carbondioxide to efi'ect substantial deflocculation to produce a drilling mudhaving a lower filtration characteristic than said untreated mud.

4. A method for treating drilling mud to produce a drilling fluid whichwill possess desirable viscosity, wall-building and water lossproperties when used in the process of drilling a well which comprisestreating said mud with carbon dioxide in an amount sufiicient to lowerthe filter rate to less than approximately 45 ml. in the first hour offiltration.

5. A method for treating drilling mud to produce a drilling fluid whichwill possess desirable viscosity, wall-building and water lossproperties when used in the process of drilling a well which comprisestreating said mud with carbon dioxide in an amount sufiicient to lowerthe filter rate to less than approximately 25 to 30 ml per hour asmeasured during the second half hour of filtration.

6. A method according to claim 2 in which the fiocculating ,agentcomprises sodium carbonate.

7. A method according to claim 2 in which the fiocculating agentcomprises sodium pyro-

